Technical Field
The present disclosure relates to a MEMS (microelectromechanical systems) resonator compensation method and a MEMS resonator, especially relates to a MEMS resonator active temperature compensation method and a thermally-actuated MEMS resonator.
Description of Related Art
With the increasing demands on the miniaturization of the electronic devices and the progress on the micromachining technology, it is possible to integrate huge amount of electronic components in a single chip. Many different components such as mechanical components, chemical components, optoelectronic components, bio system components and microfluidic components can be integrated into a single chip for achieving strong and diversified performances. In these electric components, MEMS resonators are the most representative products. Many companies have devoted into the manufacturing of the MEMS resonators.
MEMS resonators play an important role in various applications, such as sensing, actuating, and signal processing owing to excellent mechanical high Q. A general purpose of the resonator is generating frequencies which is essential in a wireless communication system, a signal processing system, or an electric circuit system. Conventional discrete electric components such as capacitance and inductance have Q factor only less than 100 which is too low to apply in an electric system for high performance, not to mention the extremely low Q of CMOS inductance and capacitance of less than 10. The mechanical resonator can provide low energy loss enabling the Q factor thereof is larger than 10000.
Therefore, mechanical resonators with high Q factor have been embedded in many electric devices such as SAW filters and quartz resonators. Main functions of the mechanical resonators are for providing frequency selection or frequency generation in high-performance oscillators or filters which are not available in conventional electric components. However, owing to too large volume and being independence from a VLSI, the conventional mechanical resonator can't be effectively integrated with an IC circuit and is not favorable for the miniaturization of an electric device.
There are three general driving methods based on electrostatic, piezoelectric or thermal-actuation mechanisms for MEMS resonators. The electrostatic driving method is to adopt an electrostatic force for driving the MEMS resonator. Operationally, when the frequency of the electrostatic force is corresponded to the resonator frequency, the displacement of the resonator structure will be magnified by Q times. As a result, a time-variant motion current is generated from the time-variant capacitance between the resonator structure and sensing electrode, therefore achieving the function of signal generation. The advantages of the electrostatic driving method are: (1) high Q factor and low energy loss; (2) easy integration with CMOS circuitries. The disadvantages of the electrostatic driving method are: (1) weak electro-mechanical coupling capability, therefore large motional impedance is induced so that it's difficult to integrate with a standard 50 Ohm electric circuit system; (2) a parasitic feedthrough effect makes it difficult to measure the weak motion current; (3) complicated fabrication processes are required to form a sub-micron capacitance gap; thereby lowering the manufacturing yield; (4) the Q factor of the resonator will be reduced owing to the influence of air damping between the sub-micron capacitance gap, so an expensive vacuum package is required.
The piezoelectric driving method has good electro-mechanical coupling capability, and the motional resistance thereof is only few ohms and is easily integrated with standard electric circuit system. The disadvantages of the piezoelectric driving method are: (1) the material of a conventional piezoelectric thin-film such as PZT and ZnO is not compatible with CMOS circuit, therefore the manufacturing processes of a piezoelectric-driving resonator is difficult to be integrated with a CMOS circuitry; (2) low Q factor and operation frequency due to intrinsic limitation of the material.
The thermal driving method has been widely utilized in the MEMS actuators. A thermal actuator has advantages on large driving force, large displacement, low operation voltage, low cost and simple processing. However, in traditional view, the thermal actuators has disadvantages on low response time and high energy loss because the designed structures are bulky, thereby requiring a sufficient time, especially in the high frequency range, to heat the large thermal capacitance for obtaining a sufficient thermal-driving force. With the progress on micro-manufacturing technology, the size of the resonator can be reduced to nano scale so that many disadvantages of the thermally-actuated resonator can be improved. The thermal time constant, for example, is shortened proportionally to the volume of the thermal beam but the mechanical operation frequency can be designed to remain the same. When the two-dimensional directions of thermal beams are scaled down in one order for each direction, the thermal-driving force is raised in the magnitude of two orders but the mechanical spring constant of thermal-beam can be the same, thus the operation frequency keeps uncharged under the same motional proof mass. That means as the miniaturization of thermal beam, the thermal-driving force under the same operation power can be raised in the magnitude of two orders for the same designed operation frequency.
However, a silicon-based mechanical resonator suffers an issue of an improper temperature coefficient of frequency (TCf), which means that the oscillation frequency of the resonator will be varied with environmental temperature so that MEMS resonator cannot exploit the real applications such as timing reference, wireless communications, GPS and so on. For example, the TCf of silicon is typically between −40 to −30 ppm/° C., which means that if the environmental temperature varied from −20° C. to 100° C., the resonance frequency shifts about 4000 ppm. Compared to the frequency stability of less than 1 ppm over the environmental temperature range from −55° C. to 125° C. requiring for mobile phones, there are still 3-order magnitudes of TCf for silicon-based MEMS resonators to achieve the commercial specification. Therefore, how to control TCf to zero or a stable value for reducing the influence of the variation of the environmental temperature is an important issue for the silicon-based MEMS resonators no matter what kind of driving mechanism is adopted.